Ge-rich graded-index SiGe waveguides as enabling building blocks for broadband mid Infrared integrated photonics

The mid-Infrared (mid-IR) photonic integrated platform is recently drawing attention due to its foreseen potential as alternative compact solution to several challenges and limitations taking place in current mainstream technologies [1]. Improved performance is expected in a wide palette of topics such as sensing, thermal imaging, nonlinear optical devices, astronomy or secure datacom, among others [2]. Up to date, different material platforms have been considered to develop mid-IR devices with functionalities beyond the state of the art. In that framework, Si and Ge have risen as promising alternative raw materials to develop mid-IR photonic devices leveraging from the higher refractive index of Ge over Si and their compatibility with the standardized CMOS platform. Also, Si and Ge possess a wavelength transparency window up to λ = 8 μm and 14 μm respectively, in concordance with the spectral range where several substances display their main spectral absorption peaks, hence opening the route towards highly-sensitive and selective label-free chemical sensors. Additionally, none of these materials experience significant nonlinear losses at those wavelengths, as Two-Photon Absorption (TPA) and other related second-order effects are strongly reduced in the mid-IR. This last point in particular, makes SiGe approaches an interesting choice to explore novel nonlinear optical devices operating in the mid-IR range, taking advantage from the commercially available high-power and largely tunable mid-IR quantum cascade lasers.